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Name of type of thermostat.

I thought initially it was a 'differential thermostat' but looking at these, does not fit the bill.

What do you call a controller that turns on/off solely on the DIFFERENCE of two sensors.

It has no setpoint in the conventional sense. It runs when sensor A registers X degrees above sensor B.

Application: I want to store cold. Insulated building. I want to run a small fan pulling outside are inside whenever the outside air temp is below the inside air temp, with the goal of freezing the top 8-10 feet of soil, as well as several hundred gallons of brine.

I'm a startup tree farmer. Our farm came with a small insulated shed that used to house the boiler for the hydronic heating system. A new boiler is in the house.

The building is R20 walls, not sure what is in the ceiling, and uninsulated slab foundation. Unmodified it remains cooler than our basement the whole summer long.

As a farmer, seedlings are time of frantic activity getting them planted in time. If stored between freezing and 40, seedlings take a LOT longer to break dormancy -- as long as 6 weeks.

So, in a nut shell, I need a refrigerator for a few weeks a year. And I almost have one. But a 12 x 12 walk in fridge is a bit pricey.

So I figured that before committing to a pile of money that I don't really have, I would try to re-purpose this building.

1. Tighter door.
2. Verify the insulation in the room.
3. Small fan under the eaves that runs whenever outside is colder than inside. I figure that I'd compare temp under the eaves with floor temp inside. Duct the fan down the wall to just above the floor. Have a vent on the other side of the room also at the eaves.

This way the room is filled with the coldest air near the floor. (The object is to get that slab, and the dirt under it as cold as possible.)

This may be sufficient. If the room doesn't remain cool long enough, I'd add a
pails of water (Pails have straight sides. As long as you have head room, on freezing the ice can slide up the walls, unlike a barrel, where the narrowing opening causes the ice to wedge, and swell the bottom.)

It may be *too* cold -- The floor may be cold enough to refreeze the trees (BAD). I can check for this by putting a milk jug of water on the floor. If the floor is that cold, then the trees have to be put on pallets or on sheets of styrofoam.

Some further control can be done by putting down a matrix of pails, a sheet of styrofoam, and the boxes of trees. Styrofoam would not go down until needed. Another fan can circulate air from below the foam to above the foam.

Eventually I'll need a proper cool room. But 30 grand and a larger electrical service can wait a few years. So far I'm clear of all debt doing this. And this may be sufficient.